This invention relates to photographic materials, more particularly it relates to photographic materials which comprises a synthetic plastic film coated with a resin and which have very few pits formed on the surface of the resin.
As a support for photographic materials, for example, lithographic printing plates, there are used metallic plates such as aluminum plate, zinc plate, etc., plastic films, paper materials, among which suitable supports are chosen considering printing endurance and other printing characteristics such as ink stains formed while printing, adhesion strength between image forming layer and support, and besides production cost, handling properties.
Recently, automatic plate making and automatic printing systems have been remarkably spread in office light printings and lithographic printing plates excellent in printing endurance and suitable for various plate-making processes have been used.
Usually, plate-making from lithographic printing plate is carried out by giving imagewise exposure from the top side of the plate (i.e. image forming layer side with reference to the support) by a process camera having a reversing mirror (often referred to as "top face exposure method" hereinafter). In this case, the support of lithographic printing plates may not be transparent and various metallic plates and paper materials may also be used as the support. However, when the plate-making is carried out by giving an imagewise exposure from the back side of the photographic material for lithographic printing plates (i.e. the side opposite to the image forming layer) by a process camera with no reversing mirror (often referred to as "back face exposure method" hereinafter), the support is required to be transparent and so metallic plates and paper materials cannot be used as the support and plastic films high in transparency are used.
The photographic materials of this invention can be used not only for the ordinary lithographic printing plates of the top face exposure type, but also for especially the lithographic printing plates of back face exposure type. The so-called back face exposure method according to which an imagewise exposure is carried out from the back side of the photographic materials by a process camera containing no reversing mirror is disclosed, for example, in Japanese Patent Unexamined Publication (KOKAI) No. 89007/73.
Because of excellent transparency, plastic films are used for various photographic materials and are also suitable as the support of photographic materials for lithographic printing plates of back face exposure type.
For supports of photographic materials for lithographic printing plates which are strongly required to have characteristics such as elongation and stiffness, polyester films are suitable although various plastic films such as triacetate films, polycarbonate films, polystyrene films, polypropylene films, polyvinyl chloride films, and the like may be used.
Photographic materials, especially those for lithographic printing plates, are used under high printing pressures and hence the surface of support and image forming layer (which might be referred to as "an emulsion layer" hereinafter) must sufficiently adhere to each other. However, due to poor surface activity of synthetic plastic films such as polyester films per se, it is difficult to directly coat the emulsion on the surface of the films and besides, even when the emulsion is coated after surface activation treatment such as corona discharge treatment, significant problems such as peeling of emulsion layer (frilling) occur during a sequence of photographic treatments due to difference in heat shrinkage of polyester film and the emulsion layer.
Adhesion between the polyester film and the emulsion layer can be increased by providing on the polyester film a resin layer of other resins such as polyolefins which are thermoplastic resins.
In general, the resin layers are provided by the melt extrusion coating method. When a polyester film is coated with a resin, generally both surfaces of the polyester film are coated with the resin when back face exposure method is employed.
As a support for lithographic printlng plate for back face exposure method, it is especially preferred in view of transparency to provide a mirror-like surface polyolefin resin layer on both surfaces of the polyester film. However, in this case, since the both surfaces are mirror-like surfaces, there is caused severe blocking (sticking between the top surface and the back surface) after having been taken-up in a reeler part and practicality is completely lost. Of course, when a back coat layer is provided in-line with melt extrusion coating, then both surfaces may be mirror-like. In order to improve the above defect, it is considered to provide roughened surface polyolefin resin layer on both surfaces of the polyester film, but in this case, the deterioration in the transparency is large due to the roughening finish and furthermore, blurs (unclear images or lines) occur and thus such materials cannot be used for lithographic printing plates for the back face exposure method. For the same reasons, materials wherein the surface on which an emulsion is coated is mirror-like and the back surface is a roughened surface cannot be used as lithographic printing plates for the back face exposure method.
Therefore, for a support of lithographic printing plates for the back face exposure method, it is necessary to provide a mirror-like surface resin layer on the back surface, namely, on the surface on which no emulsion is coated. In this case, as mentioned herein-before, the melt extrusion coating method is generally used. When, for example, the ordinary low-density polyethylene resin is coated on a polyester film by this melt extrusion method, fine pores (called "pits" hereinafter) are often formed on the surface of the coated resin. These pits cause loss of microscopic uniformity of the mirror-like surface resin layer on the back side and hence when imagewise exposure is given by the back face exposure method, light does not transmit uniformly therethrough and lithographic printing plates made therefrom have blurs (unclear images or lines).
As a result of the inventor's intensive research to improve the above problems, it has been found that a photographic material which comprises a synthetic plastic film, e.g., a polyester (abbreviated as "PET" hereinafter) film which has on one surface a resin layer comprising a resin composition containing at least 50 parts by weight (based on the total weight of the resin layer) of a polyolefin which has undergone at least one kneading, preferably a resin composition containing 50 to 95 parts by weight (based on the total weight of the resin layer) of said polyolefin and 5 to 50 parts by weight, more preferably 10 to 30 parts by weight (based on the total weight of the resin layer) of a high-density polyethylene has very few pits on the surface and is suitable especially as a support for lithographic printing plates according to back face exposure method.